Morphology Evolution of γ′ Precipitates during Isothermal Exposure in Wrought Ni-Based Superalloy Inconel X-750

The morphological evolution of γ′ precipitates and lattice misfit with isothermal aging were closely investigated in wrought Ni-based superalloy Inconel X-750. The γ′ morphology dramatically changes in terms of shape, distribution, coalescence and coherency at the γ/γ′ interface. These processes and their dependence on temperature are summarized as a γ′ morphology map together with a time–temperature–precipitation (TTP) diagram through quantifying relevant morphological parameters. The lattice misfit was measured by X-ray diffraction and is positive; it decreases from 0.6% at room temperature to 0.1% at the aging temperature. These results suggest that the morphological changes of the γ′ precipitates are attributable to very low lattice misfit, the interaction of the elastic field, the volume fraction of the precipitates and incoherence in γ/γ′ interface

Effect of Ti and Al Contents on γ′ Morphology in Wrought Ni-based Superalloys

The effect of the content of Ti and Al on the morphology of γ′ precipitates is examined for 13 kinds of Ni-based model alloys aged at 1073 K/100 h. The morphology of γ′ precipitates was evaluated using the absolute moment invariants technique. The magnitude of cuboidal for γ′ particles clearly depends on the lattice misfit, but does not depend on the volume fraction of the particles. More γ′ particles tend to form a flat and parallel interface pair or elongated shape when the Ti and Al content is increased. These results suggest that the lattice misfit makes γ′ particles cuboidal and volume fraction strengthens their interaction, leading to a unique morphology

Precipitation Behavior of Wrought Fe-Ni-Based Alloy HR6W

Age-hardening behavior of Fe-Ni-based alloy HR6W was investigated at the temperature range between 973 K and 1073 K. Two-step increase in hardness is detected for the alloy at each temperature; the first increase in hardness results from the precipitation of M23C6 phase and the second one corresponds to that of Laves phase. The TTP (time-temperature precipitation) diagram for the alloy is established based on the results of hardness measurement and microstructure observation, where the precipitation of Laves phase is slower than that of M23C6 phase by three orders of magnitude and the nose temperature of Laves phase is above 1073 K. The M23C6 phase precipitates with plate-like morphology along grain-boundaries at the early stage of aging, followed by the precipitation of Laves phase with granular morphology with increasing aging time. It is found that the M23C6 and Laves phases are aligned under stress condition, due to their precipitation on the dislocations introduced during creep deformation

Three-Dimensional Morphology of C15–Al2Ca Precipitates in a Mg–Al–Ca Alloy

The three-dimensional morphology and thickness of the C15–Al2Ca Laves phase, which precipitated within the primary α-Mg grains, were investigated for the Mg–5Al–1.5Ca alloy over-aged at 523 K for 100 h using the high-resolution transmission electron microscopy (HRTEM). The C15–Al2Ca phase precipitated with a hexagonal plate-like morphology along the (0001)α basal plane of the α-Mg matrix phase, where the sides of the hexagonal plates were parallel to the second columnar plane {1120}α of the α matrix. The typical coffee bean contrast was clearly visible around the precipitates, indicative of coherent precipitation of the C15–Al2Ca phase. The thickness of the C15–Al2Ca precipitates, which corresponds to the six layers of (111)C15 plane composed of Ca atoms, was evaluated to be approximately 1.5 nm

The Effect of Nb/Ti Ratio on Hardness in High-Strength Ni-Based Superalloys

The age-hardening behaviour and microstructure development of high strength Ni-based superalloys ABD-D2, D4, and D6 with varying Nb/Ti ratios have been studied. The studied alloys have large volume fractions and multimodal size distributions of the γ′ precipitates, making them sensitive to cooling conditions following solution heat treatment. Differential scanning calorimetry was conducted with a thermal cycle that replicated a processing heat treatment. The hardness of these alloys was subsequently evaluated by nanoindentation. The Nb/Ti ratio was not observed to influence the size and distribution of primary and secondary γ′ precipitates; however, the difference in those of tertiary γ′ and precipitate morphology were observed. The nanoindentation hardness for all alloys reduces once they have been solution-heat-treated. The alloys exhibited specific peak hardness. The alloy with the greatest Nb content was found to have the best increase in hardness among the alloys studied due to its large tertiary γ′ precipitate

The effect of Nb/Ti ratio on hardness in high-strength Ni-based superalloys

The age-hardening behaviour and microstructure development of high strength Ni-based superalloys ABD-D2, D4, and D6 with varying Nb/Ti ratios have been studied. The studied alloys have large volume fractions and multimodal size distributions of the γ′ precipitates, making them sensitive to cooling conditions following solution heat treatment. Differential scanning calorimetry was conducted with a thermal cycle that replicated a processing heat treatment. The hardness of these alloys was subsequently evaluated by nanoindentation. The Nb/Ti ratio was not observed to influence the size and distribution of primary and secondary γ′ precipitates; however, the difference in those of tertiary γ′ and precipitate morphology were observed. The nanoindentation hardness for all alloys reduces once they have been solution-heat-treated. The alloys exhibited specific peak hardness. The alloy with the greatest Nb content was found to have the best increase in hardness among the alloys studied due to its large tertiary γ′ precipitate

On the Effect of Nb on the Microstructure and Properties of Next Generation Polycrystalline Powder Metallurgy Ni-Based Superalloys

Abstract The effect of Nb on the properties and microstructure of two novel powder metallurgy (P/M) Ni-based superalloys was evaluated, and the results critically compared with the Rolls-Royce alloy RR1000. The Nb-containing alloy was found to exhibit improved tensile and creep properties as well as superior oxidation resistance compared with both RR1000 and the Nb-free variant tested. The beneficial effect of Nb on the tensile and creep properties was due to the microstructures obtained following the post-solution heat treatments, which led to a higher γ′ volume fraction and a finer tertiary γ′ distribution. In addition, an increase in the anti-phase-boundary energy of the γ′ phase is also expected with the addition of Nb, further contributing to the strength of the material. However, these modifications in the γ′ distribution detrimentally affect the dwell fatigue crack-growth behavior of the material, although this behavior can be improved through modified heat treatments. The oxidation resistance of the Nb-containing alloy was also enhanced as Nb is believed to accelerate the formation of a defect-free Cr2O3 scale. Overall, both developmental alloys, with and without the addition of Nb, were found to exhibit superior properties than RR1000.This work was supported by the Rolls-Royce/EPSRC Strategic Partnership under EP/H022309/1, EP/H500375/1 and EP/ M005607/1

Dislocation analysis of a Mg–Al–Ca alloy by transmission electron microscopy

The dislocation structure was analyzed in a Mg–4.98Al–1.46Ca (mass%) alloy using invisible criterion with transmission electron microscopy. Basal and non-basal segment of dislocation induced during die-casting is determined as much 〈a〉 and a few 〈a+c〉 dislocation. After the aging at 523 K for 10 h and tensile deformation at 473 K, only basal 〈a〉 dislocation can be observed. This dislocation structure is not observed in crept specimens, which support the difference in dominant strengthening mechanism between creep and tensile deformation